1. Field of the Invention
The invention relates to a machining unit, particularly for transfer machines, comprising a working spindle supported in a spindle housing, a saddle on which the spindle housing is removably secured and which is guided on a saddle carrier for sliding movement axially of the working spindle to cause the working spindle to be moved by feed motions, a drive motor secured to the saddle carrier and a transmission driven by the drive motor, the transmission having a hollow shaft in which the working spindle is guided for axial sliding movement and for rotation by the hollow shaft.
2. Description of the Prior Art
The U.S. patent No. 2,190,284 describes a machining unit of the above kind in which the hollow shaft is directly and stationarily supported in the saddle carrier, whereby fixed relative positions are established for the hollow shaft and guides defined on the saddle carrier, on which guides the saddle is guided for sliding movement. The sliding movement of the saddle is obtained from a double-acting piston and cylinder unit of which the piston is secured to the saddle and the cylinder is fixedly connected to the saddle carrier.
The only element which participates in the sliding movements of the saddle is the spindle housing together with the working spindle supported therein and a boring chuck fixed to the working spindle and carrying a boring tool clamped therein, whereas the drive motor and the transmission do not change their position relative to the saddle carrier. One result of this is that the axial movements of the working spindle are followed by relatively small masses of inertia only; thus, these movements admit important accelerations and decelerations. On the other hand, the spindle housing which moves together with the working spindle in the axial direction of the latter and the saddle conjointly ensure an always equally efficient support of the working spindle; consequently, resulting forces of the metal removing process which act upon the working spindle in the axial direction, on the one hand, and the feed forces transmitted from the piston and cylinder unit to the saddle, on the other hand, cannot produce but small deformations the extent of which is exactly predictable, since for a given amount of the forces they will be of the same importance in any position of the saddle. In principle, the known machining unit is thus suitable not only for boring work, but also for other metal removing operations such as turning and milling. However, the universal use of the known machining unit is opposed by the fact that the working spindle must be arranged at an exactly determined distance from the guides provided for the saddle on the saddle carrier, and that there is hardly two spindle housings to be found having mating dimensions which could be considered as being of the same value within sufficiently close tolerances. Therefore, after having replaced a spindle unit with another one, one cannot be sure that the working spindle and its driving hollow shaft will cooperate without any jamming.
Admittedly, such difficulties which prevent the replacement of spindle units with each other are avoided in machining units of the kind described in the Swiss patent specification No. 573,796 in which a working spindle is mounted axially non-slidable in a spindle housing which, in turn, is replaceably secured to a saddle which is slidable in a direction parallel to the spindle axis for obtaining the feed movements of the working spindle, the spindle housing further carrying a transmission with a drive motor flange-mounted thereto. The connection between the working spindle and the drive motor is maintained both during the replacement of the spindle unit as well as during each and every conceivable displacement of the spindle unit; one resulting advantage from this is that there is no danger of jamming between the working spindle and its driving transmission elements, but, on the other hand, one must accept the drawback which consists in that the drive motor and the transmission participate, with their relatively large inert mass, in each and every axial movement of the working spindle and thus do not allow axial displacements to be made by rapid motion under heavy accelerations and decelerations.